1. Field of the Invention
The present invention relates to an active-type automatic focusing apparatus that irradiates, with light, a subject, the range from which is intended to be found, that is, a subject of range finding, and receives light reflected from the subject to calculate information of the found range.
2. Related Background Art
Hitherto, a so-called active-type automatic focusing apparatus has been known which emits light from a light emitting device, such as an IRED, to a subject and which receives light reflected from the subject by a semiconductor position sensing device (PSD) or the like to measure the distance from the position, at which light has been received, to the subject by a principle of triangulation.
Referring to FIG. 5, an active-type automatic focusing apparatus of the foregoing conventional type will now be described.
Referring to FIG. 5(a), reference numeral 10 represents a light emission side lens, 11 represents a light receiving side lens, 12 represents an IRED (Infrared Ray Emitting Diode), 13 represents a semiconductor position sensing device (a PSD), and 14 represents a subject.
Light emitted from the IRED 12 is allowed to pass through the light emission side lens 10 and the subject 14 is irradiated with light. Light reflected by the subject 14 is allowed to pass through the light receiving side lens 11 before it is made incident upon the PSD 13. The position at which light is made incident upon the PSD 13 is shifted depending on the change in the distance from the subject 14, thus causing electric currents I.sub.A and I.sub.B transmitted from pair of the two ends of the PSD 13 to be changed.
Thus, an apparatus of the foregoing type is arranged in such a manner that the electric currents I.sub.A and I.sub.B are electrically amplified to calculate the distance to the subject.
A problem of the contrast of the subject will now be described.
If the subject 14 has no contrast, the center of a spot 15 of light reflected by the subject 14 is brought to a central position of the spot 15, a distance x.sub.1 from an end of the PSD 13, as shown in FIG. 5(b). Thus, the level of each of the output electric currents I.sub.A and I.sub.B at the two ends of the PSD 13 corresponds to the position x.sub.1.
Assuming that the contrast, and thus the reflectance of the right half portion of the subject 14 is low, the light spot 15 on the PSD 13 has a contrast as shown, for example, in FIG. 5(c). That is, the position of the center of the light spot 15 on the PSD 13 is shifted to right by a distance a. As a result, the position of the center of light is made to be (x.sub.1 +a), thus causing distance information corresponding to the shift to be displaced in the direction in which the distance is shortened.
As described above, the conventional active-type automatic focusing apparatus has a basic problem that the apparatus erroneously measures the distance from a subject if the subject has a contrast. The foregoing problem also arises in a case where the irradiation beam is applied to only a part of the subject.
An arrangement that is capable of overcoming the foregoing problem has been disclosed in U.S. Pat. No. 4,814,810. FIG. 6(a) illustrates the basic structure of the disclosure of the '810 patent in such a manner that the same elements as those shown in FIG. 5 are given the same reference numerals.
The active-type automatic focusing apparatus according to the '810 patent comprises a light receiving side lens 16 at a symmetrical position with respect to the light emission side lens 10 in a direction of the base line of the light receiving side lens 11. The active-type automatic focusing apparatus further includes a PSD 17 at the position of the focal point of the light receiving side lens 16. Reference numeral 12 represents an IRED.
If the subject 14 has, for example, a contrast as shown in FIG. 6, light emitted from the IRED 12 is reflected by the subject 14. As a result, spot images are formed on the PSDs 13 and 17. A state of the spots is shown in FIG. 6(b).
Since the light receiving side lenses 11 and 16 are positioned symmetrically with respect to the light emission side lens 10, adequate adjustment of the positions of the PSD 13 and the PSD 17 enables the spot positions x.sub.1 and x.sub.2 to be symmetrical and to have the same values.
If the subject 14 has the contrast as shown in FIG. 6(a), the state of spot images on the PSDs 13 and 17 is as shown in FIG. 6(b). Assuming that the difference in the position of the center of light taking place depending upon the contrast of a subject 14 is a, the position of the center of light is, on the PSD 13, (x.sub.1 +a) and that on the PSD 17 is (x.sub.2 -a) because the center of light moves in the same direction on the PSDs 13 and 17.
Thus, output F.sub.1 from a distal terminal of the PSD 13 (an end of the PSD 13, the output from which is made to be larger than another terminal when a subject is positioned away from the apparatus) and output F.sub.2 from a distal terminal of the PSD 17 are enabled to be added to each other by connecting the foregoing terminals. Output N.sub.1 from a proximate terminal of the PSD 17 (a terminal of the PSD 17, the output from which is made to be larger than another terminal when a subject is positioned at a near position) and output N.sub.2 from a proximate terminal of the PSD 17 are enabled to be added to each other by connecting the foregoing terminals. Then, electric currents I.sub.F and I.sub.N denoting the results of the additions and expressed as follows are taken: ##EQU1##
Thus, the output electric currents I.sub.A (I.sub.N) and I.sub.B (I.sub.F) are not dependent on the spot center movement, a, caused by the contrast of the subject.
As described above, because the two PSDs 13 and 17 are positioned away from each other by the length of the base line and disposed symmetrically with respect to the light emission side lens 10, and because the output sum from each of the PSDs 13 and 17 is obtained the influence of the subject contrast is basically eliminated. However, the apparatus disclosed as described above involves a necessity of further disposing the light receiving side lens and the light receiving device with respect to a center of symmetry in the direction of the base line as compared with the conventional active-type automatic focusing apparatus. Therefore, there arises a problem in that the size of the apparatus cannot be reduced.